Adding to this lovely discussion: Aren't lightsabers powered by the force? which would explain why whenever you see a jedi/sith defeated his/her lightsaber always sheaths. So really there is no chance of it going off while on you belt.

(someone may have said this, didn't read all the posts)

No, some are though but most have buttons. Otherwise Han wouldn't have been able to use Luke's saber to cut open the tauntaun on hoth.

"There is one lesson you've yet to learn. How to become one with the Force!"
―Cin Drallig to Darth Vader